Power supplies often substantially determine the size of electronic circuits or devices. Due to this reason, compact and highly efficient planar transformers have been developed.
For example, the U.S. Pat. No. 6,069,548 (Nokia) discloses a planar transformer constituted by a supplementary circuit board surrounded by a magnetic core. The planar transformer is conventionally surface mounted on a motherboard by a plurality of pins soldered to tracks on the motherboard.
One of the limiting factors in the design and application of such transformers is heat removal. In many cases the amount of power that can be processed through a transformer is purely dependent upon the thermal limitations imposed by the transformer. Often, a physically larger transformer is used purely because it can dissipate more heat than a smaller one due to its larger surface area, despite the fact that the larger transformer generates more losses than the smaller one.
The conventional method of removing heat from a transformer circuit consists in mechanically attaching a heatsink to the magnetic core and/or the windings. This heat sink usually covers only part of the core or the windings and has to be electrically isolated. Apart from the extra expense involved, the improvement in heat transfer is marginal.
The U.S. Pat. No. 6,239,683 B1 (Tyco Electronics Logistics A.G.) describes a flat transformer that is surface mounted on a substantially planar substrate by means of a plurality of relatively massive conductive posts. The posts are electrically connected to the transformer windings and may support the removal of heat from the transformer circuit.
However, the removal of heat is only effective in local areas surrounding the places of attachment of the posts. Furthermore, the joints between the posts and the transformer need a higher melting point than the joints with the substrate in order to prevent melting during final assembly. The posts add unwanted inductance and resistance and increase the total assembly height.